INTRODUCTION: This study assessed the effect of different light sources and light guides on the shear bond strength and failure site of orthodontic brackets bonded with 2 adhesive systems (Quick Cure [Reliance Orthodontic Products, Itasca, Ill] and Transbond XT [3M/Unitek, Monrovia, Calif]). METHODS: Ninety bovine permanent mandibular incisors were randomly divided into 6 groups according to the adhesive system and light-curing procedure used. Each group consisted of 15 specimens; 90 stainless steel brackets were bonded to the teeth. Each adhesive system was light-cured with a halogen light (Ortholux XT, 3M/Unitek) for 20 seconds and the conventional light guide as controls. The remaining groups were cured with a halogen light (Ortho 2000 [Reliance Orthodontic Products, Itasca, Ill]) and a Power Slot light guide (Reliance Orthodontic Products) for 6 and 10 seconds. After bonding, all samples were stored in distilled water for 24 hours and then tested in a shear mode on a universal testing machine. RESULTS: No significant differences in terms of bond strength values were found among the 6 groups, but significant differences in debond locations were found among the 6 groups tested. CONCLUSIONS: Power Slot light guides can be recommended as advantageous alternatives for fast-curing composite resins during orthodontic bonding.